1. Field of the Invention
This invention relates to diaphragm devices for cameras, and more particularly to a diaphragm drive method in the diaphragm device with a stepping motor as the drive source for driving the plurality of blades stepwise to vary the size of aperture opening.
2. Description of the Related Art
The diaphragm device in which the plurality of blades are driven by the stepping motor as the drive source to adjust the diameter of aperture opening are advantageous in many respects. For example, the structure is simple and the control is also easy, because the number of driven steps of the stepping motor can be related directly to the number of stops of the diaphragm.
By the way, in general, in the diaphragm device having the plurality of blades (for example, 5 to 6 blades), as the diameter of the aperture opening decreases, the overlapped area of each blade on the adjacent two ones increases. Therefore, the friction of all the blades is much increased, causing the mechanical error to be one-sided. As a result, the accurate adjustment tends to become difficult. For example, when the diaphragm blades move in a direction from a relatively large diameter of the aperture opening to smaller diameters, it will happen that the resultant diameter of the aperture opening is slightly larger than the aimed one because of the one-sidedness of the mechanical tolerance due to the aforesaid increase of the friction occurs. Conversely in the region of smaller diameters of the aperture opening, when the change of the diameter from the presently given value to a slightly larger one is carried out, the resultant diameter will be slightly smaller than the aimed one.
Such an error in the adjustment of the diameter of the aperture opening, if taking place in the region of larger diameters, does not give rise to a very serious problem, for the rate of error is small. In the region of smaller diameters, on the other hand, since this rate becomes large, it becomes serious on the point of view of assuring a correct exposure.